Hi all,
I wish you a Merry Christmas; it's almost gone ), and a Happy New Year with many puzzles of the very hard one type. I like them very much and I always learn new tricks to manage them. By the way the Christmas one from today is the same as from 25.12.05! :). They are marvelous. I could use all the technics like triplets, quadruplets, X-wing, XY-wing.
Just one question. When you have the following pattern is this also a xy-wing? I have learned that XY-wings goes like this:

XY

Code:

XZ

YZ

Code:

no Z

(with only two digits and not three ones!)

box 3: 89

Code:

box 6: 789

box 7: 47

Code:

box 9: 79

Then I could eliminate the "7" in box 9. The boxes are counted clockwise.
This pattern happened in the christmas tree sudoku puzzle on 23.12.05.
Thanks a lot in advance! Your German sudoku freak